1. Field of the Invention
The present invention relates to an imaging apparatus which receives light.
2. Description of the Related Art
Cameras with built-in lenses are required to be smaller, and to record an image of an object at a position as close as possible to the camera. Accordingly, instead of mechanically moving a correcting lens and a variator lens in association with each other with a cam, a so-called inner-focus system is commonly used. In this system, the correcting lens is moved on the basis of lens cam data stored in a microcomputer in advance and representing trajectories of the correcting lens, and focusing is performed using the correcting lens.
FIG. 10 is a diagram showing the structure of a known inner-focus lens system. With reference to the figure, the system includes a fixed front lens 901, a zoom lens (also called a variator lens) 902 used for zooming (first lens unit), a diaphragm 903, a fixed lens 904, a focusing lens 905 (second lens unit) which serves as a correcting lens having a focus adjustment function and a function of correcting the displacement of an image plane caused by zooming (a so-called compensating function), and an imaging surface 906.
In the lens system shown in FIG. 10, the focusing lens 905 serves both the compensating function and the focus adjustment function. Therefore, even when the focal length is constant, the position of the focusing lens 905 for focusing on the imaging surface 906 varies depending the object distance. FIG. 11 is a graph obtained by plotting the position of the focusing lens 905 for focusing the object image on the imaging surface 906 versus focal length for different object distances. When zooming is performed, a trajectory corresponding to the object distance is selected from a plurality of trajectories shown in FIG. 11, and the focusing lens 905 is moved along the selected trajectory. Accordingly, the focused state is maintained during zooming.
In a lens system in which focusing is performed using a front lens, a focusing lens is provided separately from a zoom lens and the zoom lens and the focusing lens are mechanically connected to a cam ring. Accordingly, when the focal length is changed by manually rotating the cam ring, the lenses are reliably moved by the cam ring no matter how fast the cam ring is rotated. Since the zoom lens and the focusing lens move along an optical axis while sliding along cams formed on the cam ring, image blurring due to zooming does not occur as long as the focusing lens is at an in-focus position.
In comparison, in the above-described inner-focus lens system, information of the trajectories shown in FIG. 11 or information corresponding thereto (information representing the trajectories or functions taking lens position as a parameter) is stored in advance, and zooming is performed by moving the focusing lens along a trajectory selected from among the trajectories on the basis of the positions of the focusing lens and the zoom lens.
As is clear from FIG. 11, when the zoom lens is moved in the direction from telephoto to wide angle, the focused state can be maintained using the above-described trajectory tracing method since the trajectories converge toward the wide-angle end. However, when the zoom lens is moved in the direction from wide angle to telephoto, the trajectory to be traced by the focusing lens cannot be determined if the focusing lens is at a position where the trajectories converge, and therefore the focused state cannot be maintained by the above-described trajectory tracing method.
Accordingly, Japanese Patent No. 2795439 discloses a control method in which the focusing lens is repeatedly moved in a direction causing the image to go out of focus and then in a direction to adjust the focus on the basis of the information representing the focus state (in other words, the moving speed is varied) when the zoom lens is being moved for zooming. In addition, a method for increasing the accuracy of selecting the trajectory to be traced is also disclosed in Japanese Patent No. 2795439 (FIGS. 3 and 4). According to this method, the period of variation in a sharpness signal is varied by changing the amount of variation in a tracing speed depending on the object distance, the focal length, and the depth of field.
In the above-described control method for the zooming operation, focus detection is performed by a TV-AF method using a video signal from an imaging device. Therefore, processes are normally performed in synchronization with a vertical synchronizing signal.
On the other hand, as is clear from FIG. 11, when zooming is performed using the inner-focus lens system, the cam trajectories to be traced by the focusing lens are on substantially the same point at the wide-angle end for object distances in the range of several tens of centimeters to infinity. Therefore, when the TV-AF method is used, the cam trajectory to be traced cannot be selected accurately unless the zoom lens is moved to an area near the telephoto end.
In TV-AF, a signal detection period at which an AF evaluation value is obtained is equal to the period of the vertical synchronizing signal. Accordingly, as the zoom speed increases, the accuracy of determining the trajectory to be traced is degraded. Therefore, although actuators for focusing and zooming have recently been improved and made smaller, and inexpensive super-high-speed actuators have been developed, the potential of such actuators cannot be sufficiently exploited when the inner-focus lens system and the TV-AF method are used in combination, and hence, there is a limit to the zoom speed. Although high-speed zooming using super-high-speed actuators can be performed when the angle of view is adjusted in the standby mode, the zoom speed must be reduced in the recording mode in order to prevent image blurring.
In addition, when long-time exposure (recording), such as so-called slow shutter, is performed, the detection period of the AF evaluation value becomes equal to the exposure period, and the tracing accuracy is degraded even when the zoom speed is not high. Therefore, image blurring may occur when the trajectory is being determined and a long time is required for correcting the image blurring if zooming and panning are performed simultaneously. As a result, the imaging performance is degraded.
In addition, also when the contrast of the object is low or when the signal-to-noise (S/N) ratio is low due to low illumination, in the zooming operation, an accurate AF evaluation value cannot be accurately detected by the TV-AF method. Therefore, the trajectory-tracing performance is also degraded in these cases.